Forestry in the name of climate change

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As the world seeks ways to cut atmospheric carbon dioxide (CO2) -- the greenhouse gas produced by burning fossil fuels -- science says managed forests will play a key role.

Trees are the most powerful concentrators of carbon on Earth. Through photosynthesis, they absorb CO2 from the atmosphere and store it in their wood, which is nearly 50 percent carbon by weight.

You might be surprised to learn young forests outperform old growth in carbon absorption. Although old trees contain large amounts of carbon, their rate of absorption has slowed to a near halt. A young tree, although it contains little fixed carbon, pulls CO2 from the atmosphere much faster.

While cutting down an old tree results in a net release of carbon, new trees growing in their place can more than make up the difference. Wooden furniture made in the Elizabethan era still holds the carbon fixed hundreds of years ago.

The relationship between trees and greenhouse gases is simple enough on the surface. Trees grow by taking carbon dioxide from the atmosphere and, through photosynthesis, converting it into sugars. The sugars are then used as energy and material to build cellulose and lignin, the main constituents of wood.

When a tree rots or burns, the carbon in the wood is released to the atmosphere. Active forest management, such as thinning, removing dead trees, and clearing debris from the forest floor very effectively reduces the number and intensity of forest fires. And the removed wood can be put to good use for lumber, paper and energy.

Accounting provides a useful metaphor to discuss forests and carbon sequestration. Old growth forests often have a large "balance" of carbon that has built up over time in wood and soil. They don't add much new carbon because they decay at about the same rate they grow.

In financial terms, this is like a company with many assets that operates on a break-even basis. Young forests have a smaller balance of carbon compared to old forests but accumulate carbon rapidly. In that sense, they are like an emerging, very profitable company with few assets that is growing rapidly.

The effect of forests on the global carbon cycle can be boiled down to these key points:

On the negative side, the most important factor influencing the carbon cycle is deforestation. This results in a permanent loss of forest cover and a large release of CO2 into the atmosphere. Deforestation -- primarily where tropical forests are permanently cleared for agriculture and urban settlement -- accounts for about 20 percent of global CO2 emissions, says a United Nations-World Meteorological Organization Intergovernmental Panel on Climate Change report.